There are many processes that can benefit from the ability to precisely vector a jet and to control its width. These include thin-film coating processes in which it is vitally important that the film thickness be uniform, even if the surface to be coated is not flat. In many of these processes, such as thermal sprays, the contents of the jet or spray are combusting, making the environment in which the jet operates very hostile. Control schemes that rely on vectoring of the jet nozzle would place moving parts in this hostile environment, where they would wear quickly, and be severely limited in slew rate. Multiple nozzles can be used to cover a large area, but they (or the coated surface) must be traversed. Additionally, it is difficult to coat evenly in this manner.
The most fundamental method of changing the direction and shape of a jet is by modifying the direction and shape of the nozzle from which it emanates. The hardware required to effect these changes is unreliable and heavy and thus slow.
More elegant methods include the use of secondary flows to modify the jet. One method is to use oscillatory blowing to vector a planar jet. A high slew rate is one of the primary advantages to this method. However, it is difficult to reliably generate the required oscillatory blowing. Another method uses a combination of blowing and suction through adjacent slots to achieve a similar effect. Suction combined with a Coanda surface has been shown to be effective for vectoring a compressible jet flow. Schemes involving suction prohibit use in hostile environments such as combustion since hot and/or corrosive gas would be drawn into the suction slot.
There is considerable need for a nozzle that can be built over a large range of scales, operate in a hostile environment, and position a jet or aerosol precisely and at high slew rate.
Many industrial spray processes can benefit from precise direction and profile control. thermal spray processing is an established industrial method for applying “thick coatings” of metals (stainless steel, cast iron, aluminum, titanium and copper alloys, niobium and zirconium) and metal blends, ceramics, polymers, and even bio-materials at thicknesses greater than 50 micrometers. Several different processes, including Combustion Wire Thermal Spray, Combustion Powder Thermal Spray, Arc Wire Thermal Spray, Plasma Thermal Spray, HVOF Thermal Spray, Detonation Thermal Spray, and Cold Spray Coating can benefit from the ability to alter the direction of the spray. Currently, expensive robots are commonly used for this purpose. Thermal spray coatings are used for corrosion and erosion prevention, chemical or thermal barrier and wear protection, and general metalizing on applications ranging from aircraft engines and automotive parts to medical implants and electronics. The process involves spraying molten powder or wire feedstock onto a prepared surface (usually metallic) where impaction and solidification occur. Melting typically occurs through oxy-fuel combustion in the nozzle or an electric arc (plasma spray) located just downstream of the nozzle structure. Thermal spray processes typically result in very high material cooling rates (>106 K/s). Similarly, Flame Spray Pyrolysis (FSP), a process to synthesize metal and mixed metal oxide nanoparticles, uses a flame as an energy source to produce intraparticle chemical reactions and convert liquid sprayed reagents to the final product. Due to the high temperature combustion environment present in or near these process nozzles, mechanical vectoring of the nozzle is not feasible since this would place moving parts in the jet flow, reduce device durability, and severely limit directional frequency response. Furthermore, traversing a part to be coated, which is often heated to high temperatures, is costly.
Films are deposited on surfaces (substrate) using a variety of thermal spay processes, depending on the material to be deposited and the surface on which it is to be applied. The processes generally belong to one of three categories: flame spray, electric arc spray, and plasma arc spray. The nozzles of modern thermal spray devices are designed to create the desired process and are generally not directional. Coating of large surfaces is achieved by traversing the spray gun, sometimes with a dedicated robot.
In many flame processes, as little as 10% of the flame energy is used to melt the feedstock. This results in excessive heating of the substrate. The time that the coating material resides in the flame, termed residence time, is critical to many characteristics of the coating, including porosity and oxidation. Porosity of the coating is very important and is a function of many parameters of the process, including particle speed, size distribution and spray distance. Molten material that is not sufficiently heated may result in higher porosity, as can sprays applied at a large angle relative to the surface. In many applications, it is desirable to have low porosity, while in others, higher porosity may be beneficial (e.g. tribological applications and biomedical implants). Thus, a robust and simple method to control porosity is beneficial.
One method of changing the direction and shape of a jet is by modifying the direction and shape of the nozzle from which it flows. This is currently being investigated as a method for thrust-vectoring of fighter aircraft, although the hardware required to effect these changes is unreliable and heavy (and thus slow). More elegant methods include the use of secondary flows to modify the jet. High frequency response is one of the primary advantages of this method. However, it is difficult to reliably generate the required oscillatory blowing. Suction combined with a Coanda surface has been shown to be effective for vectoring a compressible jet flow. Unfortunately, schemes involving suction prohibit use in hostile environments such as combustion since hot and/or corrosive gas would be drawn into the suction slot.